Colour sensation is a unique physiological reaction of the human eyes to light stimulation. It is generally held that there exist three different kinds of cone cells on the retina of the human eye, which sense red, green and blue colours, respectively. Those with normal colour vision, sometimes referred to as normal trichromats, have cone cells which properly sense these three colours. However, colour vision deficiencies are created when one type of cone cells is completely defective, or all three types of cone cells are partially defective.
A complete defect of one kind of cone cells results in two colour sense. This condition is known as red-green colour blindness, and persons having this condition are called dichromats.
Dichromats can be divided into two subgroups, protanopes and deuteranopes. Both protanopes and deuteranopes perceive the colour spectrum as being blue, neutral and yellow. In the case of protanopes, the average neutral point is about 490 nm, and the average brightest point is about 540 nm, compared to about 555 nm with individuals with normal vision. In the case of deuteranopes, the average neutral point is about 500 nm, and the brightest point is about 560 nm.
A partial defect of all three types of cone cells also causes abnormal three colour sense, particularly when colours are in low saturation. This condition, known as red-green colour deficiency, is less severe than red-green colour blindness. Individuals suffering from this lesser condition are called anomalous trichromats, because while they can see all three colours, the matching process differs from normal trichromats.
There are two types of anomalous trichromats, protanomals and deuteranomals. Protanomals have "red-weak" colour sense, because they sense less red than normal trichromats. Deuteranomals have "green-weak" colour sense, because they sense less green than normal trichromats.
About 8% of the male population are believed to suffer from red-green colour blindness or colour vision deficiency. There is no known cure for hereditary colour blindness.
Over the years, however, various attempts have been made to create visual aids, which increase the ability to discriminate between colours. See Ingeborg Schmidt, "Visual Aids for Correction of Red-Green Colour Vision Deficiencies", Can J Optom 1976; 38:38-46, for a history survey. These visual aids typically take the form of coloured filters which assist in distinguishing between certain colours, by introducing differences in their brightness, colour or vividness, without enabling a colour-blind person to perceive colours in the normal fashion.
One of the earliest known visual aids for colour blindness comprised a red filter and a green filter which enabled those having red-green colour vision deficiency to distinguish some colours better by viewing them alternately through the filters. Other early visual aids utilized a pair of spectacles. In one case, a pair of spectacles having one green glass and one red glass allowed some colours to be distinguished by first closing one eye and then the other. In another case, the pair of spectacles included a green gelatin filter mounted in the upper half of the frame, which left the lower half clear. Single red filters, as well as single green filters, have also been found to help some colour-blind people. Magenta colour filters have been used to enhance chromaticness, i.e. to increase the quality of certain colours by enhancing their vividness or intensity. A more recently developed visual aid is the "X-Chrom" lens, a contact lens worn by colour deficient individuals on their non-dominant eye.
These prior art visual aids have not, however, achieved wide acceptance. While coloured filters allow individuals to distinguish colour differences indirectly through differences in brightness and so on, they do not create colours that have not been perceived before. Filters tend to reduce the number of colours that can be perceived. Most filters also reduce the overall amount of light entering the eye, making them unacceptable for use in low lighting conditions. Those visual aids which are worn in front of only one eye, such as the X-Chrom lens, are said to have certain deleterious effects, such as distorted depth perception. Colour vision aids utilizing colour filters, such as a pair of glasses with one green lens and one red lens, are generally not regarded as being acceptable or pleasing.